The present invention relates to a lamella for use in vulcanization molds for producing a vehicle tire. The lamella has projections and depressions extending substantially parallel to the peripheral surface of the tire tread of the vehicle tire to be produced. Each projection has a highest point-shaped location and each depression has a lowest point-shaped location whereby the projections and the depressions are substantially arranged in a uniform area division. The areal division is defined by linear (line-shaped) dividing lines positioned preferably centrally between the highest point-shaped locations and the lowest point-shaped locations. The dividing lines are comprised of sets whereby the dividing lines within each set extend parallel to one another and the dividing lines of different sets intercept one another. The projections may also have a line-shaped (linear) highest location while the depressions still have a lowest point-shaped location or, in the alternative, each projection may have a highest point-shaped location while the depression has a line-shaped (linear) lowest location. In this case, the dividing lines are either positioned at the highest line-shaped location or the lowest line-shaped location.
The present invention also relates to a vulcanization mold having lamellas as disclosed above.
Also, the invention relates to a vehicle tire having a tire tread with sipes that are produced by lamellas embodied as disclosed above.
The inventive lamella is to be used in vulcanization molds (2) whereby such vulcanization molds (2) are used for producing vehicle tires (3). The inventive lamellas are arranged in the area of the tire tread to be molded in order to produce in the tire tread sipes having a design that matches, with the exception of minimal uniform shrinkage, the negative pattern of the lamella design.
Even though the greatest economic value of the invention lies within the inventive vehicle tire with the novel sipes, in the present application the inventive lamella will be disclosed first because with it the complicated special shape of the lamella of the present invention can be more easily shown than the sipes of a tire tread.
Even though the rubber mixture of the tire tread during molding of the sipes by lamellas is still plastic and shrinkage of the rubber during cooling from the vulcanization temperature to the tire operating temperature is minimal and is substantially uniform in all three special directions, the design of the sipe matches substantially exactly the shape of the lamella that has produced this sipe. The sipe and the lamella producing this sipe behave, with the exception of minimal shrinkage, as a positive and a negative pattern so that their edges are substantially inverse congruent, i.e., each body edge of the lamella projecting counter to a direction of viewing has a negative edge extending in the same direction which is a avoid area edge within the same projection counter to that same direction of viewing and each body edge of the lamella with a projection in the direction of viewing has a negative edge extending in the same direction within the same projection. This means that by disclosing the shape of the lamella the shape of the sipes in the tire is also disclosed.
It is known that such sipes in tire treads serve to soften the tire tread and also provide edge formation in order to thus increase grip on slippery surfaces. Such sipes are especially used in large numbers for snow tires.
It is also known that the degree of softening depends approximately with the third power from the depth of the sipe. This results in the problem for smooth sipes that the cut tread surface when the tire is new, especially when a sipe depth is identical to the depth of the tread grooves, the tread surface is unnecessarily soft and has an unnecessary degree of edge formation with disadvantages during dry handling, respectively, with respect to wear.
Even though a reduced depth of the sipes eliminates such disadvantages, another disadvantage is produced, i.e., after wear of a tread depth corresponding to the reduced sipe depth, sipes are no longer present. Accordingly, the tire will loose its grip on slippery surfaces.
An improved solution is known according to which for a full sipe depth the effectiveness of the areas located deep within the sipe is greatly limited in that the sipes are wave-shaped or curved so that positive-locking engagement between the closely positioned walls of the same sipe will result. According, only small radially outer areas of the sipes are effective whereby these effective areas with increased wear will move radially inwardly so that the softening effect, by propagation of the effective deformation base to the actual sipe base, will fluctuate less.
Conventionally sipe widths in car tires are between 0.4 to 1.0 mm, whereby especially the width of approximately 0.6 mm is used for car tires. Car tires are the preferred application of the invention. Inasmuch as such sipes are used for truck tires, commercial truck tires or even heavy truck tires, the required width increases approximately with the root of the suggested tire pressure and linear with the depth of the tire tread. For heavy truck tires with approximately four times the tire pressure and twice the tread depth, suitable sipe widths are between 1.6 and 4.0 mm, preferably 2.4 mm.
The known suggestions, to provide after a limited deformation travel positive-locking engagement as a function of the width of the sipe, can be divided into three groups.
A first group comprises the suggestions of wave-shaped sipes. In contrast to the aforementioned depressions and projections of the prior art, upon which the inventive design is based, all projections have a line-shaped (linear) highest location, usually referred to as a peak, even more precisely referred to as a crest (BK), and all depressions have a line-shaped (linear) lowest location, usually referred to as a valley, even more precisely referred to as a valley bottom (TS). Such suggestions can be taken from FIG. 2 of French Patent 791 250, European Patent application 0 564 4 35, German Patent application 44 27 895, Austrian Patent 401 160 and still unpublished German Patent applications 196 50 702.2 and 197 10 400.2 whereby the latter shows crests and valleys that are curvilinear while the others show straight lines or angled lines.
A second group includes suggestions having curved sipes wherein all projections have a line-shaped (linear) highest location and all depressions have a point-shaped lowest location or visa versa. Such a suggestion is disclosed in Great Britain patent 1,150, 295.
A third group includes suggestions having sipes that are curved such that all projections have a point-shaped highest location, referred to as a top (G), and all depressions (5) have a point-shaped lowest location, referred to as a crater (K). This is disclosed in published PCT document WO 96/01189. Based on the conventional mounting of the lamellas in the vulcanization mold, such that the produced sipes in the tire tread extend substantially axially and radially, the measuring direction for the height of the projections and the depth of the depressions extends thus substantially parallel to the peripheral surface of the tire tread of the produced vehicle tire. The depth of the depression is to be understood as a negative height. Accordingly, for the deformation height and deformation depth (more precisely the dimple depth) the same reference numeral can be used: Z.
In all suggestions of the prior art the crests or peaks of a sipe, respectively, of a lamella are positioned in a common plane and at the same time all valleys or craters of the same sipe, respectively, of the same lamella are positioned in another common plane whereby both planes extend parallel to one another. At least for the suggestions of the first and third group, it appears to be expedient to define a reference plane between these two planes. The reference plane is used to measure the deformation heights and depths, and it is referred to in the following as the Z=0 plane.
In the two following paragraphs the image is discussed which results when a lamella is cut along this reference plane (Z=0 plane). The resulting section lines are such that their height/depth (Z) extends centrally between the height of the highest location of the projections and the depth of the lowest location of the depressions. It appears to be expedient to define this height/depth Z as 0.
In the known suggestions of the first group, the produced section lines are straight or curved (DE 197 10 400). They all look alike and extend to one another so as to be staggered in parallel so that they do not intercept one another. The spacing between these lines corresponds to half the period length (repeating unit length).
It is thus expedient to define the thus generated lines as dividing lines (T). The division according to these suggestions is thus only one dimensional, i.e., perpendicular to the dividing lines.
In the known suggestions of the second group for section lines selected as above disclosed, isolated, aligned diamonds result i.e., quadrangles having identical sides. The aligned sides of these diamonds describe exactly two sets of parallel extending, straight, uninterrupted lines. Within each set of such lines, the lines extend parallel to one another. The lines of different sets intercept one another.
The periodicity (repetition pattern) of this suggestion can be illustrated in an even more simple manner when as a reference plane the plane is selected within which the line-shaped (linear) extremes are positioned, whereby it does not matter whether this extreme is the crest or the valley bottom. This results in linearly contacting, aligned diamonds, i.e., quadrangles having identical sides. The aligned sides of these diamonds are defined by two sets of parallel extending straight, uninterrupted, i.e., continuous, lines. Within each set of such lines, the continuous lines extend parallel to one another. The lines of different sets intercept one another. It appears to be expedient to define the thus generated lines in the following as dividing lines (T). The division in this suggestion is thus two-dimensional, i.e., areal. The spacing between neighboring parallel dividing lines corresponds to the period length.
For a discussion of the known suggestion of the third group the section plane is expediently positioned between the plane containing the tops and the plane containing the craters. The section then shows contacting and aligned rectangles or squares. The aligned sides of these quadrangles are comprised of two sets of parallel-extending, straight, uninterrupted, continuous lines, which are defined in the following as dividing lines. Within each set of lines, the continuous dividing lines extend parallel to one another. The lines of different sets cross (intercept) one another. This periodic division is also two-dimensional, i.e., areal. The spacing between neighboring parallel dividing lines corresponds to half the period length.
It is desired that for the inventive lamella design the sequence of depressions and projections is to be provided within a substantially uniform areal division, whereby the aforementioned areal division is to be defined by substantially straight dividing lines (T) having a height/depth (Zt; whereby expediently Zt=0) that is preferably centrally arranged between the height (Zg) of the top (G) and the depth (Zk) of the crater (K) or is positioned at the height (Zbk or Zts) of the line-shaped extremes, i.e., the crests (BK) or the valley bottom (TS).
The dividing lines (T) are to be arranged in at least two sets whereby the dividing lines (T) within each set extend parallel to one another and the dividing lines (T) of different sets intercept (cross) one another.
It is known that with respect to the tire properties the sipes are to be as thin as possible.
The inventors have realized that the optimal lamella shape not only must take into consideration the tire properties to be effected directly, but also the load on the lamellas itself. The lamellas must be so thick that their buckling and bending can be avoided reliably. While the most favorable thickness of the lamellas has in the past been determined by trial and error, the invention is based on the following analysis.
The lamellas are loaded by radial pressure and thus with regard to buckling during the profile-generating final lift of the green tire after closing of the mold and before begin of the vulcanization. When producing tires having a final lift that is smaller than the sipe depth, the lamellas upon contacting the green tire periphery with the edges forming the base of the sipe are also loaded with radial and circumferentially acting pressure and thus with respect to buckling and bending within the segment border areas.
It is therefore an object of the present invention to provide the shape of a lamella which provides in addition to excellent positive-locking engagement of the thus produced positive surfaces limiting the sipe a greater bending stiffness and thus an especially increased buckling stability relative to the lamella thickness.
Based on the aforementioned features of the prior art, this object is solved by providing dividing lines (T) in three sets whereby the dividing lines (T) of different sets intercept one another at an angle of approximately 60xc2x0 so that a grid is defined that is comprised of substantially equilateral triangles. The invention is based on the recognition that triangles, especially equilateral triangles, will result in much greater stability in comparison to quadrangles when used as the basic shape of a frame work or truss structure.
When considering the points of interception between the xe2x80x9crigidxe2x80x9d stays or rods of a planar frame work as joints with a pivot axis perpendicular to the plane of the frame work, it is shown that such rod triangles are stable while quadrangles are not. Quadrangles are in principle compressible and expandible along their diagonals. Accordingly, the deformed and thus bending-soft walls of the lamellas of the prior art suggestions of the second and third group are loaded even for the smallest load in the radial direction of the tire to be produced by bending forces. Even the points of interception themselves, contrary to the aforementioned hypothesis of pivot ability, are loaded by bending forces. Accordingly, such lamellas will buckle even for relatively small radial loads relative to the thickness of its wall.
In the inventive design on the other hand the sheet metal areas in the vicinity of the three sets of straight dividing lines act as a stable planar framework within the Z=0 reference plane. A degree of freedom with regard to buckling thus is present only in a direction perpendicular to the Z=0 reference plane, i.e., substantially in the peripheral direction of the tire to be produced when based on the conventional, substantially axial alignment of the sipes in plan view onto the tire tread. There is no degree of freedom with regard to buckling within the Z=0 reference plane.
Accordingly, the inventive lamellas are buckling-stable up to higher radial forces relative to the wall thickness of the lamella. This allows reduced wall thickness of the lamella having the effect on the tire properties that, even for a minimal tread block deformation, the walls, that delimit a sipe and are very close to one another, will contact one another and thus produce the desired positive-locking engagement. This provides the inventive tire with a dry handling that is close to that of excellent summer tires. Furthermore, slight advantages with respect to wear can be observed without negatively affecting handling on mud or slippery surfaces.
The prior art embodiment according to which all crests or tops of the sipes, respectively, of a lamella are positioned in one plane and all valleys or craters of the same sipe, respectively, the same lamella are positioned in another common plane, parallel to the first mentioned plane, can also be used in connection with the present invention and is disclosed in with respect to multiple embodiments.
However, an even more refined dimensioning of the lamella wall thickness is possible synergistically with the embodiment of a stable framework in the Z=0 reference plane when the reference plane Z=0 is not exactly planar but, in a plan view onto the tire tread to be manufactured, corresponding to a plan view onto the end face of the lamella to be connected to the vulcanization mold, is embodied as a cylinder mantle portion or, more preferred, in an angled embodiment as a prism mantle portion. With respect to the preferred curvature or angled embodiment, the invention does not refer to the Z=0 reference plane, but in a more general term of the Z=0 reference surface.
Since the grid stays between the points of interception of the framework are to be as buckling-resistant as possible with respect to their width and thickness, they are preferably not curved but straight. It is also preferred to concentrate the curvature onto the points of interception of the framework. This results in a design according to which the bending lines of the Z=0 reference surface coincide with at least some, more preferred all, dividing lines (T) of one of the three sets of dividing lines whereby these bending lines, in a more preferred embodiment, extend substantially radially relative to the finished tire. A bending angle of 3xc2x0 between bent portions already results in a considerable stabilization. Bending angles greater than 30xc2x0 appear to be unnecessary.
The angled embodiment of the Z=0 reference surface can also be zigzag-shaped, i.e., not monotonous but changing in its orientation, for example, alternating. This allows for employment of large bending angles.
Due to the high stability of the inventive triangular structure of the non-deformed or substantially non-deformed lamella areas the lamella wall thickness for car tires can be below 0.4 mm, whereby for an angled extension of the Z=0 reference surface, when viewed in a plan view, it can be taken to below 0.3 mm, and for an even more angled extension, meaning bending angles greater than 7xc2x0, even thinner.
The mentioned thickness of the sheet metal refers to the used blank of sheet metal. The finish-stamped lamella sheet metal pieces have this wall thickness only in the area of the Z=0 reference surface. The deformed portions have, due to drawing effects upon stamping, correspondingly reduced wall thickness.
For an angled extension of the Z=0 reference surface all of the crests or tops of a sipe, respectively, a lamella are positioned in a curved or angled or wave-shaped or zigzag-shaped first surface. Preferably, all valleys or craters of the same sipe or the same lamella should be positioned in a curved or angled or wave-shaped or zigzag-shaped second surface whereby preferably these first and surfaces are to be parallel to one another.
The inventive measure of angling the reference surface transforms the areally stable framework portions to a three dimensionally stable framework so that the remaining bending ability that is still possible for a planar reference surface, i.e., bending perpendicular to the Z=0 reference surface, is lowered.
The design and the advantages of the triangular framework of the inventive lamellas are especially prominent when the dividing lines are positioned within narrow areas, which may be referred to as stays or rods, which are not deformed during the stamping process. The parallel base lines (B) of adjacent (neighboring) pyramid-shaped projections or depressions have a spacing of greater than 0 so that they appear as optionally rounded or angled body edges. The width (b) of the stay surfaces (S) between parallel base lines of neighboring pyramids should be smaller than 40% of the edge length (base line) of the pyramid base surface, preferably approximately 20%.
The width (b) of the aforementioned stay surfaces (S) should be at most twice the thickness of the stay, whereby the stay thickness in the simplest manufacturing process, stamping, is substantially identical to the thickness (wall thickness) of the sheet metal blank from which the lamella is produced.
In the following the design of the inventive lamellas and sipes will be disclosed in more detail.
For variants of the invention having depressions and projections with a point-shaped highest or lowest location, it is expedient to define a reference surface substantially centrally (for non-parallel arrangements: along the bisecting line) between the two surfaces in which the extremes (tops/craters) are positioned, preferably at a location where uninterrupted, continuous lines are formed, that is the dividing lines. This reference surface is used to define all of the height and depth values of the tops and the craters. This surface in this context is referred to as the Z=0 reference surface. Inasmuch as the surfaces containing the extremes (tops and craters) are parallel to one another, the Z=0 reference surface is also parallel to these two surfaces.
For variants of the invention wherein the projections have a highest line-shaped location or the depressions have a lowest line-shaped location, the reference surface is to be defined within the plane in which all of the line-shaped highest locations or line-shaped lowest locations of the projections or depressions are positioned. This surface is referred to in the context of this invention as the Z=0 reference surface.
In addition to the variants discussed above, i.e., having all deformations projecting with their tips in one orientation (schematically represented by ++++++++) or alternating in the two possible orientations relative to the Z=0 reference surface (schematically represented as +xe2x88x92+xe2x88x92+xe2x88x92+xe2x88x92), it is possible to also have combinations of such arrangements, for example, represented schematically by +xe2x88x92xe2x88x92xe2x88x92+xe2x88x92xe2x88x92xe2x88x92.
The latter embodiment of the invention is a combination of a) projections and depressions with point-shaped highest and lowest locations and having a triangular pyramid shape and b) projections and depressions having line-shaped highest or lowest locations either at the depression or the projection in combination with point-shaped highest or lowest location at the projection of depression, respectively. This inventive combination has a Z=0 reference surface containing the three sets of uniformly intercepting sets of substantially straight dividing lines (T) forming a grid of equilateral triangles, whereby in the direction of at least one bisecting line of the triangles differently oriented deformations are positioned having an orientation sequence which differs from that of +xe2x88x92+xe2x88x92+xe2x88x92+xe2x88x92 and is instead preferably represented by +xe2x88x92xe2x88x92xe2x88x92+xe2x88x92xe2x88x92xe2x88x92 whereby the positive deformations are projections and the negative deformations are depressions or vice versa.
Such long repeating patterns (periodicity), in the last mentioned embodiment four dividing lengths, are to be recommended only for very small dividing lengths in comparison to the sipe depth. At least two such repeating lengths should fit into the sipe depth.
The axis of the repeating pattern to be selected as desired should be the bisecting line of the triangular base surface.
In a preferred embodiment, each one of the projections (4) of the inventive lamella (1) should be embodied as a pyramid projecting from a base surface that is an equilateral triangle and may have rounded edges or sharp edges. Each one of the depressions (5) of the inventive lamella (1) should be embodied as an inverse pyramid having a base surface that corresponds approximately to an equilateral triangle whereby the pyramid may have rounded or sharp edges.
When the height of such a pyramid is identical to the square root of ⅙ (approximately 0.4082) times the base line length (edge length) and when the base lines of neighboring deformations coincide, i.e., have the spacing 0, and when the orientation (projection or depression) of the deformations along the bisecting line of the base surfaces alternates, then this results in cubes positioned alternatingly onto their tips, i.e., an alternating sequence of cube inner surfaces and cube outer surfaces. Expediently, however, with respect to a sufficiently easy removability of the tire from the vulcanization mold, the pyramid height is selected to be smaller, preferably to approximately 30% of the base line length (edge length).
The section line between a lateral surface of such a pyramid and the base surface of such a pyramid within the exactly planar or curved or angled Z=0 reference surface is called base line (B).
The sipes produced with lamellas according to the first embodiment, respond in the same manner to acceleration and deceleration forces, i.e., they are not unidirectional, when all three base lines (B) of all pyramids are positioned centrally between the height (Zg) of the top (G) and the depth of the (Zk) of the crater (K).
In contrast to all known prior art suggestions, it is also possible to position the Z=0 reference surface outside of the center plane/surface between the plane containing all tops and the plane containing all craters. This results in that the magnitude of the depth of the crater is smaller than the magnitude of the height of the top or vice versa. This results in a unidirectional behavior of the sipes in the tread surface. The embodiments in which the Z=0 reference surface coincides with one of the surfaces containing the extremes (tops or crests or craters or valleys) exhibit also a unidirectional behavior for the same reasons.
Such unidirectional behavior can be employed especially for rear wheel driven vehicles where the wheels at the rear axle are to be provided with excellent grip in the forward direction and wherein the wheels at the front axle should be embodied substantially so as to have good braking behavior. Expediently, such a unidirectional design of the sipes is combined with a known unidirectional design of the other elements of the tread of the tire, for example, with an arrow-shaped arrangement of transverse grooves.
The especially important repeating unit (period length) in the schematic representation +xe2x88x92+xe2x88x92+xe2x88x92+xe2x88x92 has the advantage that a unidirectional embodiment is avoided for shortest possible repeating units (period lengths), i.e., two dividing lengths. This means, on the other hand, that the dividing length can be selected to be relatively large, for car tires preferably approximately 3 mm. Dividing lengths that are too small will result in supporting surfaces that are too small and thus provide less stiffening. The lamellas of such alternating repeating units are characterized in that at each one of the three base lines (B) a triangular pyramid-shaped projection (4) has positioned adjacent thereto a triangular pyramid-shaped depression (5) with a respective base line (B) and that at each one of the three base lines (B) of a triangular pyramid-shaped depression (5) a triangular-shaped projection (4) is positioned with its respective base line (B).
The lamellas with non-alternating repeating units, represented schematically by ++++++, have a Z=0 reference surface with unchanged dividing lines positioned within the plane that contains the line-shaped highest or lowest location because the dividing lines themselves will become the line-shape extremes.
The inventive lamellas are designed to be mounted in the inner mold chamber of a vehicle tire vulcanization mold at the surface that forms the tread of the tire. It is not necessary that all of the lamellas of the vulcanization mold are embodied according the present invention. It is instead possible to employ a combination of the inventive lamellas with conventional lamellas of different types. It is especially preferred to employ the inventive lamellas in those mold areas which produce the positive shoulder patterns of the tire. In comparison to the lamellas disclosed in German Patent application 196 50 702 especially suitable for the central portion of the tire tread, the inventive sipes provide an especially high transverse force transmission which can be used with advantage in the shoulder area of the tire.
For the same reasons, the vehicle tires produced by employing the inventive lamellas must not be provided at all sipes with the negative pattern of the inventive lamellas.
The special feature of the inventive vehicle tire is that at least some of its sipes are provided with deformations in the form of depressions and/or projections arranged in such an areal repeating unit division, that all of the dividing lines are arranged in three different sets whereby within each set the dividing lines extend parallel to one another and whereby the dividing lines of different sets intersect one another at an angle of approximately 60xc2x0 so that the dividing lines form a grid consisting of substantially equilateral triangles. Preferably, the deviations from the aforementioned ideal angle of 60xc2x0 is smaller than 5xc2x0.